1. Field of the Invention
This invention concerns water-distribution piping for cleaning water in which gas is dissolved. More specifically, this invention pertains to water distribution piping for gas-dissolved cleaning water, which is capable of distributing cleaning water in which self-decomposing gas is dissolved from a main pipe to branch pipes, having multiple use points over a long distance, while keeping the gas concentration levels at all use points essentially constant.
2. Background of the Invention
Removal of foreign particles from the surface of electronic materials, such as silicon bases for semiconductors, glass bases for liquid crystal, and quartz bases for photomasks, is extremely important to ensure high product quality and low defect rates. Wet cleaning is widely practiced for this purpose. Use of cleaning fluids which have a strong oxidation power is effective for the removal of organic and metal contaminants. Conventionally, high temperature cleaning has been performed using such fluids as a mixture of sulfuric acid and hydrogen peroxide (SPM cleaning fluid) and a mixture of hydrochloric acid, hydrogen peroxide and ultrapure water (SC2 cleaning fluid). In recent years, such wet cleaning processes were questioned because of the tremendous cost of chemicals, the cost of ultrapure water for rinsing, waste fluid treatment cost, and the cost of air conditioning to ventilate chemical vapors and manufacture clean air, as well as ease the affect on the environment, due to enormous water usage, massive chemical disposal, and the release of exhaust gas to the atmosphere, all of which accompany the conventional cleaning methods.
Earlier, the inventors developed gas-dissolved cleaning water for cleaning electronic materials by dissolving ozone in pure water. Cleaning water, made from pure water in which ozone is dissolved, demonstrates extremely powerful oxidation in spite of the low concentration of dissolved ozone, which amounts to only several milligrams per liter. The water is used in processes in which contaminants, namely organic and metal impurities that adhere to the surface of electronic materials, are removed, and other processes in which the surface of silicon bases is oxidized evenly to create a layer of chemical oxidation film. Cleaning water in which ozone is dissolved leaves no residues, and thus keeps the surface of cleaned items cleaned.
This water also has an added advantage of being reusable as it becomes high purity water again after ozone is decomposed or removed. However, dissolved ozone in ozone-dissolved cleaning water self-decomposes over time and becomes oxygen gas. For this reason, distribution of ozone-dissolved cleaning water through a lengthy pipe has been considered to be impractical because of the difficulty of maintaining and controlling the ozone concentration.
In response, the inventors discovered that it is possible to suppress a drop in ozone concentration and distribute water over a long distance by mixing ozone-containing gas and pure water inside the distribution pipe while they are being delivered, and proposed the ozone-dissolved cleaning water supply system, shown in FIG. 1. From an oxygen gas tank 1 and a nitrogen gas tank 2, a mixture of oxygen gas and a minute amount of nitrogen gas is sent to a silent discharge ozonizer 3 to manufacture a gaseous mixture of ozone and oxygen gas, which is sent by an ejector and a pump, etc., into pure water, manufactured by using an ion exchanger, membrane equipment and ultraviolet ray oxidation equipment, etc., at ozone dissolution equipment 4. The gaseous mixture of ozone and oxygen gas mixes with pure water to create a gas-liquid mixture. Ozone dissolves in water to form ozone-dissolved cleaning water which, in the same gas-liquid state, flows down a main pipe 5. Ozone which dissolved in water turns into oxygen gas through self-decomposition. The amount of ozone reduction resulting from self-decomposition is replenished as ozone that is in the gas phase dissolves in the water phase. The result is that the ozone concentration in the water stays essentially constant. Ozone-dissolved cleaning water is taken from branch pipe 6, and consumed at use point 7 after vapor and liquid are separated, by either the gas-liquid separation equipment in a buffer tank 14 or using the property of bubbles to collect at the top part of the pipe. Surplus ozone-dissolved cleaning water which is not taken out from branch pipes is first passed through ozone decomposition equipment 8 to decompose and remove ozone in the water and gas phases, and then led to gas-liquid separation equipment 9 to separate the gas phase from the liquid phase. The gas phase is released to the atmosphere as exhaust gas, and the water phase is recovered as waste water, processed as necessary and reused. This ozone dissolved cleaning water supply system has made it possible to deliver water over a distance of more than 100 m.
This ozone-dissolved cleaning water supply system was developed assuming a simple distribution route, such as one in which ozone-dissolved cleaning water is delivered directly to a use point from the main pipe through a branch. With an increase in the size of electronic material factories in recent years, however, a need has arisen to transport ozone-dissolved cleaning water from the main pipe to branch pipes which have multiple use points. Branch pipes can extend from the main pipe in a number of different ways depending on factory layout. Moreover, branch pipes may, in some situations, need upstands or down pipes, due to locations of other machinery. These variations cause fluctuations in the gas-liquid mixture ratio between ozone-containing gas and pure water inside branch pipes. A problem has thus arisen in that it was difficult to supply cleaning water that has uniform ozone concentration.
It is an object of this invention to offer water-distribution piping for gas-dissolved cleaning water which can supply cleaning water, with a nearly constant gas concentration, at all use points even when the cleaning water in which a self-decomposing gas is dissolved is transported over a long distance from the main pipe to branch pipes having multiple use points in a wet cleaning process of electronic materials.
As the result of assiduous research work to solve the above-described problem, the inventors discovered that the installation of an in-line mixer at a point immediately upstream of a branching point at which a branch pipe extends from the main pipe makes it possible to control the gas-liquid mixture ratio between the gas and pure water inside the branch pipe, and supply cleaning water that has a uniform gas concentration.
This invention includes the following:
(1) Water distribution piping for gas-dissolved cleaning water which transports cleaning water, made by dissolving gas in pure water, in the presence of gas, and which has an in-line mixer immediately upstream of the point at which a branch pipe extends from the main pipe;
(2) The above-described water distribution piping for gas-dissolved cleaning water, wherein the gas is a gaseous mixture of ozone and oxygen gas; and
(3) The above-described water distribution piping for gas-dissolved cleaning water, wherein the gas-liquid mixture ratio of the fluid which flows down branch pipes is controlled by adjusting the mixing function of in-line mixers and the directions in which branch pipes extend from the main pipe.